Para-xylene is an important aromatic hydrocarbon, for which the worldwide demand is steadily increasing. The demand for para-xylene has increased in proportion to the increase in demand for polyester fibers and film and typically grows at a rate of 5-7% per year.
Major sources of para-xylene are the aromatic streams derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These complex streams are initially fractionated to separate a C8-containing fraction, which is then fed to a para-xylene production loop where para-xylene is recovered, generally by adsorption or crystallization, and the resultant para-xylene depleted stream is subjected to catalytic conversion to isomerize the xylenes back towards equilibrium distribution. Xylene isomerization can be conducted under gas phase or liquid phase conditions, but increasingly liquid phase operation is preferred to reduce ring loss and by-product formation.
Aromatic streams derived from naphtha reforming and thermal cracking generally contain undesirable olefinic hydrocarbon contaminants, including mono-olefins, dienes, and styrenes, which can adversely affect the downstream para-xylene production process and so must be removed or reduced to very low levels. Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by their Bromine Index (BI) and have typically been removed from aromatic streams, such as BTX streams, by contacting the streams with acid-treated clays. Other materials, e.g., zeolites, have also been used for this purpose. For example, U.S. Pat. No. 6,368,496 discloses a process in which a BTX stream derived from reformate is initially treated with a clay or hydro-treating catalyst to selectively remove dienes and then the resulting essentially diene-free effluent stream is contacted with a crystalline molecular sieve material, such as MCM-22, to selectively remove mono-olefins.
U.S. Pat. No. 7,744,750 discloses a process for reducing the Bromine Index of an aromatic hydrocarbon feedstock wherein the process comprises (a) pretreating the feedstock with a catalyst or absorbent to reduce oxygenates-oxygen in the feedstock to less than 5 wppm and then (b) contacting the pretreated feedstock with a catalyst comprising a molecular sieve having an MWW structure type, such as MCM-22. The effluent from step (b) may then be fed to a para-xylene production process. See also U.S. Patent Application Publication No. 2013/0324780.
Recently, there has been increasing interest in developing additional sources of para-xylene. For example, the benzene and/or toluene component of reformate can be used to produce additional para-xylene by reaction with methanol or dimethyl ether over a para-selective catalyst, such as severely steamed ZSM-5. An example of such a process is described in U.S. Pat. Nos. 6,423,879 and 6,504,072.
One problem associated with using the alkylation of benzene and/or toluene with methanol and/or dimethyl ether as a source of para-xylene is that the alkylation product inevitably contains a variety of oxygenate by-products as a result of side reactions of methanol with itself and the various aromatic species present. Such oxygenate by-products include water, alcohols, ethers, ketones, aldehydes, acid and phenolic impurities and, depending on their boiling point, are either returned to the alkylation reactor in recycle streams or leave the process through one or more product streams. In particular, the para-xylene-rich product stream tends to contain phenolic impurities, such as phenol, methyl phenols and dimethyl phenol, at levels from ten to several hundred ppmw. At such high levels, these phenolic impurities are unlikely to be processed in the para-xylene separation unit or the xylene isomerization unit and so may build up in the xylene production loop. Such a build-up not only occupies excess capacity in the paraxylene separation unit, thereby reducing its efficiency, but can also contaminate the paraxylene product and/or the para-xylene depleted residual stream which then can result in reduced performance of the xylene isomerization unit. Oxygenates may also potentially lead to equipment fouling.
There is therefore a need for an effective process for removing olefinic unsaturated hydrocarbon contaminants, such as styrenes, and oxygenate impurities, such as phenolic compounds, from xylene-containing streams, particularly those produced by the alkylation of benzene and/or toluene with methanol and/or dimethyl ether. Similar requirements apply to bio-derived aromatics feedstocks, since these typically contain trace levels of both oxygenate and olefinic impurities.